Tag Archives: thiamine monophosphatase

Grodno State Agrarian University; Institute of Biochemistry of Biologically Active Compounds, National Academy of Sciences of Belarus; e-mail: a_makarchikov@yahoo.com

Thiamine monophosphatase (ThMPase) is an enzyme of thiamine metabolism in animals whose molecular nature has still to be elucidated. In this study we have achieved a 714-fold purification of a soluble enzyme possessing ThMPase activity from a chicken liver extract. In addition to ThMPase, acid phosphatase activity was traced during purification. Both activities proved to have coincident elution profiles at all chromatographic steps implying the same enzyme involved. The molecular weight of the enzyme was 18 kDa as estimated by gel filtration. Along with ThMP and p-nitrophenyl phosphate, the purified enzyme was capable of hydrolyzing flavin mononucleotide as well as phosphotyrosine. Subcellular distribution of ThMPase activity was also explored indicating its cytosolic localization. The results of the present work imply the involvement of low molecular weight acid phosphatase in thiamine metabolism in the chicken liver.

In animals, thiamine monophosphate (TMP) is an intermediate on the path of thiamine diphosphate, the coenzyme form of vitamin B1, degradation. The enzymes involved in TMP metabolism in animal tissues are not identified hitherto. The aim of this work was to study TMP hydrolysis in chicken liver. Two phosphatases have been found to contribute to TMP hydrolysis in liver homogenate. The first one, possessing a maximal activity at pH 6.0, is soluble, whereas the second one represents a membrane-bound enzyme with a pH optimum of 9.0. Membrane-bound TMPase activity was enhanced 1.7-fold by 5 mM Mg2+ ions and strongly inhibited by levami­sole in uncompetitive manner with Ki of 53 μM, indicating the involvement of alkaline phosphatase. An apparent Km of alkaline phosphatase for TMP was calculated from the Hanes plot to be 0.6 mM. The soluble TMPase has an apparent­ Km of 0.7 mM; this enzyme is Mg2+ independent and insensitive to levamisole. As estimated by gel filtration on a Toyopearl HW-55 column, the soluble enzyme has a molecular mass of 17.8 kDa, TMPase activity being eluted simultaneously with peaks of flavinmononucleotide and p-nitrophenyl phosphatase activity. Thus, TMP appears to be a physiological substrate for a low-molecular weight acid phosphatase, also known as low-molecu­lar-weight protein phosphotyrosine phosphatase.